Purified isophthalic acid is a precursor material for the manufacture of various polymeric materials including unsaturated polyesters. It is derived from less pure "crude" isophthalic acid by purification which can be accomplished by dissolving the crude isophthalic acid in a feed solution comprising a polar solvent at a high temperature, reducing the temperature to effect crystallization of the isophthalic acid, and separating the purified isophthalic acid crystals from the aqueous solution.
Another method of purification of crude isophthalic acid utilizes hydrogen and a noble metal catalyst such as described in Meyer, U.S. Pat. No. 3,584,039 and Stech et al., U.S. Pat. No. 4,405,809 for purification of crude terephthalic acid. The purification process involves dissolving crude isophthalic acid in hot deionized water, contacting the solution with hydrogen as a reducing agent and passing the solution over a fixed bed catalyst containing a noble metal such as palladium on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915 for the purification of crude terephthalic acid, incorporated by reference herein. The isophthalic acid solution is then cooled to a temperature which effects crystallization of the isophthalic acid, which is separated from the solution. More recently, in Schroeder et al., U.S. Pat. No. 4,933,492, which is incorporated by reference herein, an improved isophthalic acid purification process has been described which utilizes a catalyst system comprising at least two of palladium, platinum, rhodium, ruthenium, osmium, and iridium-containing components supported on active carbon carrier particles.
Optimally, the isophthalic acid purification process lowers the levels of organic intermediates, primarily 3-carboxybenzaldehyde (3-CBA), and inorganic contaminants and colored compounds. The color level of purified isophthalic acid is generally measured directly either by measuring the optical density (O.D.) of solutions of purified isophthalic acid or the b*-valve of the solid purified isophthalic acid. The optical density of purified isophthalic acid is measured as the absorbance of light at 340 and 400 nm in a solvent such as sodium hydroxide or ammonium hydroxide.
The measurement of the b*-value of a solid on the Hunter Color Scale is described in Hunter, The Measurement of Appearance, Chapter 8, pp. 102-132, John Wiley & Sons, N.Y., N.Y. (1975), and in Wyszecki et al., Color Science, Concepts and Methods, Quantitative Data and Formulae, 2d Ed., pp. 166-168, John Wiley & Sons, N.Y., N.Y. (1982). A specific description of measuring the b*-value of isophthalic acid is described in the above-mentioned U.S. Pat. No. 4,933,492.
The overall effect of the purification process for isophthalic acid is to convert impurities which normally crystallize with isophthalic acid into compounds which can be separated from the isophthalic acid by pratical techniques. After separation of the purified isophthalic acid crystals, all of the remaining mother liquor stream, which comprises isophthalic acid and impurities dissolved in water, is typically directed to a waste treatment unit for disposal. However, it is advantageous to minimize waste streams due to environmental and economic concerns.
Before this invention, it was believed that in order to maintain purity requirements, mother liquor from an isophthalic acid purification process must be discarded to the waste stream. For example, in the purification by hydrogenation over noble metal catalysts of other aromatic polycarboxylic acids such as terephthalic acid, the practice of recycle has been prohibited due to limitations in the level of the impurity, p-toluic acid in the purified terephthalic acid. Surprisingly, it has been found that up to about 60 percent of mother liquor in an isophthalic acid purification process can be recylcled to the feed stream without significant reduction in quality of the isophthalic acid product as measured by optical densities and b*-valve. This discovery permits a significant reduction in the waste stream. In a preferable process according to this invention, about 20 percent to about 60 percent of the mother liquor can be recycled without significantly affecting product quality. A typical recycle amount would be about 25 percent. This recycle offers environmental advantages including reducing water consumption in the feed stream and reducing the generation of waste-water and the total organic carbon sent to the waste-water treatment unit.